JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKIT. T 
centration des im Theilungsstück befindlichen Protoplasmas,” must be 
considered — like the corresponding one in regard to the equality or 
inequality of cleavage produets— as an example of the immature 
generalizations to which embryology has fallen heir through the acci- 
dental circumstance that the amphibian egg was for a long time the chief 
object for the study of cell division. It seems to be exceedingly dit- 
cult to grasp the fact, every day becoming more evident, that because a 
statement is true for the eggs of some of the lower vertebrates it does 
not follow that it must be true for the cells of all organisms. 
(4) The rhythm of cleavage has an important relation to the other 
processes of morphogenesis. If eleavage took place coincidently in all 
the cells of the egg, the latter still retaining its form, there could 
apparently be no “rotation” of the cells upon one another, and conse- 
quently no gastrulation. The tension in all directions would be the 
same; none of the cells would be moulded to fit the extension of 
neighboring cells, and all would retain approximately the positions held 
at the beginning 
It would perhaps be possible to carry this into detail, and show that 
the earlier cleavage of the large ventral cell, leaving it in a resting 
condition, and therefore plastic (as indicated by a comparative study of 
the forms taken by resting and by dividing cells) when the other cells 
divide and extend, is directly favorable to gastrulation. 
The question as to the faetors determining the time and the equality 
or inequality of eleavage, ib will be seen, does not at present admit of 
any direct simple answer. Cleavage is a part of the process of morpho- 
genesis, and its rhythm and other features are related to the nature of 
the form to be produced. 
D. As to Differentiation accompanying Cleavage. 
The facts bearing upon this question to be derived from the observa- 
tion of the early development of Asplanohna are few in number. The 
principal phenomenon to which attention must be directed is that of the 
segregation and migration of the cloud of granules, described on pages 
25, 30, 37, and 54. As will be remembered, a concentration of granules 
begins in the ventral region of one of the cells at the eight-cell stage 
(Fig. 7) and becomes more and more marked as successive cleavages 
take place, till a well defined cloud of very large and distinct spherules 
occupies the anterior and ventral margin of the cell d* in the stages 
immediately succeeding the sixteen-cell condition (Fig. 32, Plate 4, and 
Fig. 48, Plate G). Then oceur the remarkable migrations shown in 
